The present invention relates to a refractory element used for the transfer of molten metal. A particular case wherein the invention is very advantageous is a refractory tube for the transfer of steel from a ladle to a tundish and particularly such a tube when used without preheating.
Refractory elements used in molten metal casting are by nature, extremely sensitive to thermal shocks. When they are used, the elements contact metal and are subjected to important thermal shocks generating the formation of cracks, and this all the more when the temperature is low before their use. Consequently, the life time of these elements is reduced. Moreover, the cracks can permit air entries which can lead to downgrading the cast metal quality.
In order to improve the thermal shock resistance of the elements, a widely spread technique consists in preheating the element to temperature as close as possible to the use temperature. However, this technique requires to have a preheating zone near the use zone of the elements, consumes energy and consequently is expensive. Further, there is a minimum preheating time before which the element is not enough preheated to resist a thermal shock and a maximum preheating time beyond which the element starts to deteriorate. This process lacks also some flexibility since it does not allow to face an unexpected event, or too important deviation with respect to manufacturing planning.
Another technique well known by the skilled in the art and combined with that above described is the use of insulating fibers which are either glued, either cemented on the outside of the refractory element. In this case, the external coating permits to keep longer the heat acquired during the preheating and to increase its efficiency. However, the fibers which can support high temperatures (>1000° C.) necessary in these applications are toxic and their use is less and less authorized.
Document DE 38 05 334 A1 discloses another method permitting to improve the thermal shock resistance of such elements. This method consists in introducing in the pouring orifice of the element a sleeve made from a fibrous or foaming ceramic material. This method has several drawbacks. When a foaming ceramic material is used, to form it, it is necessary to use foaming or tensioactive-agents which are generally incompatible with refractory elements, particularly if they are constituted from carbon bonded material. It can also be difficult to control the foam so as to form a layer of relatively constant thickness and showing reproducible insulating properties. The so obtained insulation is thus not homogenous and can cause detrimental temperature gradients inside the element. When the element has a complex geometry, which is more and more frequent to improve the cast metal quality, the manufacturing and the positioning of the sleeve is specially uneasy, in particular to ensure a continuous contact between the sleeve and the element. As the sleeve is not integral with the element, it can move or even come off during the handling or usage of the element when contacting the metal. Parts of the sleeve can obstruct the element, form a plug or, at least, make uneasy the passage of molten metal since the metal cannot flow normally in the lower metallurgical vessel; it can then leak through the joints bonding the refractory elements to one another.
In the particular case of refractory pouring tube, intended for the transfer of a molten metal from a casting ladle to a tundish, these being generally tubes made from graphite based materials and carbon bonded (alumina/graphite, magnesia/graphite, . . . ), the most often used method is certainly the one consisting in pre-oxidizing the inner surface of the tube so as to form a layer without or with only a reduced carbon percentage. This low carbon content oxidized layer is a layer that shows a low thermal conductivity coefficient with respect to the body of the tube. It acts as a barrier at the beginning of the casting and permits to the refractory tube to resist the thermal shock of the first contact with the molten metal. This method, although generally satisfactory, has nevertheless some drawbacks. The oxidized layer is obtained during the firing of the refractory tube under oxidizing atmosphere. It is therefore quite uneasy to obtain an homogenous layer of constant thickness all along the element. The thickness of the oxidized layer can vary significantly (2 to 10 mm) from one tube to another or from one region to another of the same tube. This does not permit to have homogenous insulating properties. Further, this layer having lost its carbon binder is washed away in a few minutes at the contact of the molten metal. The thickness of the tube is therefore quickly reduced of the thickness of the layer; this reduces significantly the mechanical resistance and its time life.
The object of the present invention is a casting element having an increased thermal shock resistance and which does not have the drawbacks of the above mentioned prior art. Moreover, it would be desirable to propose a refractory element having improved properties, particularly a gas permeability significantly reduced with respect to the element of the state of the art.